Bacterial Pathogenesis: An Overview

II. BACTERIAL PATHOGENESIS

A. AN OVERVIEW

The overall purpose of this Learning Object is to introduce the concept bacteria virulence and pathogenicity.

LEARNING OBJECTIVES FOR THIS SECTION

Bacteria are found in almost every environment. Only a relatively few bacteria cause human disease and many benefit humans. For example, many are important decomposers that assure the flow and recycling of nutrients through ecosystems. Others have important industrial and pharmaceutical uses. However, in this section we are going to concentrate on bacteria that are potentially harmful to humans and try to understand what factors influence their ability to cause disease.

Pathogenicity and virulence are terms that refer to an organism's ability to cause disease. Technically, pathogenicity (def) is used with respect to differences between microbial species whereas virulence (def) denotes differences between strains of the same species. In practice they are often used interchangeably.

To cause disease, an organism must:

1. Maintain a reservoir before and after infection (humans, animals, environment, etc.),

2. Leave the reservoir and gain access to the new host,

3. Colonize the body, and

4. Harm the body.

Anything the bacterium does to aid in the above will influence its ability to cause disease and they are able to do these things primarily as a result of their structures and their metabolic products. We must keep in mind, however, that whether or not a person actually contracts an infectious disease after exposure to a particular potentially pathogenic bacterium depends not only on the microorganism but also on the number of bacteria that enter the body and the quality of the person's innate and adaptive immune defenses. (Innate and adaptive immunity will be discussed in detail later in Units 4 and 5.)

For example, if relatively few bacteria enter the body then the body's natural defenses against infection have a much better chance of removing them before they can colonoize, multiply, and cause harm. On the other hand, if a large number of bacteria enter then the body's defenses may not be as successful. Likewise, a person with good innate and adaptive immune defenses will be much more successful in removing potentially harmful bacteria than a person that is immunocompromized. In fact a person highly immunosuppressed, such as a person taking immunosuppressive drugs to suppress transplant rejection, or a person with advancing HIV infection, or a person with other immunosuppressive disorders, becomes very susceptible to infections by microorganisms generally considered not very harmful to a healthy person with normal defenses.

However, in this section we are going to look at bacterial virulence factors that can influence its ability to cause infectious disease. These virulence factors will be divided into two categories:

1. Virulence factors that promote bacterial colonization of the host, and
2. Virulence factors that damage the host.

For further information onthe general concepts of bacterial pathogenesis, see the online Microbiology Web Textbook at the University of Wisconsin-Madison.

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